Multilateration systems are used to provide, in particular, the position of aircraft in flight or on airport runways. A signal transmitted by a transmitter on the aircraft is received by a number of receiver stations at known locations. The signal is transmitted by a 1090 MHz Secondary Surveillance Radar (SSR) transponder and is one of a number of known code types or formats Mode A/C and mode S. By comparing the time of arrival of the signal at each of the receiver stations and with the knowledge of their locations it is possible to calculate the position of the aircraft at the time of transmission. Such a system and a multilateration technique is described in UK patent document GB2250154.
It will be appreciated that in order to accurately determine the position of an aircraft, it is necessary to account for variations in apparent path length between parts of the system, so that a meaningful difference in time of arrival values for the signal can be derived. This can give rise to a factor called group time delay. Aging of components, for example, may cause a variation in the group time delay which may require servicing of parts of the system or other remedial action or correction.
In addition, the group time delay will have an effect on the accuracy of the calculated position of the target. This is called a Geometric Dilution of Precision (GDOP). GDOP will vary depending on the position of the target.